The instant invention relates generally to film monitoring and, more particularly, to an improved method and apparatus for optically monitoring thin flims for simultaneously determining the thickness and the degree of uniformity.
The apparatus and methods of the invention were specifically developed for determining parameters of thin radiation-transmissive films of material that may be formed as self-supporting sheets or as coatings on a base sheet. By way of example, a thin film of an organic material, such as polyethylene, may be formed on a relatively thick base sheet formed from paper or metal foil. Typically, a great many metal cans, such as aluminum cans, have applied to one or more of their surfaces a protective organic resin film such as above-described. To perform their functions adequately as protective barriers, it is necessary that these films have a certain critical minimum thickness. To insure the existence of this minimum thickness, it is common practice to apply a coating in its wet state in greater than necessary amounts as a sort of safety factor. Quite obviously, the application of this excess coating is uneconomical and leads to fabrication difficulties. A characteristic of the film with which the invention is concerned is that the film formed either as a self-supporting sheet or as a coating, has specular surfaces and will be strongly reflective of radiation incident to either a first surface or second surface, including a second surface which is the interface surface between the film and an underlying base sheet. Radiation incident to the film will be reflected at the first and second surfaces and, in the case of a reflection-type measurement, the reflected component subsequently detected by electrical radiation sensors connected in a circuit to provide an output signal or readout that is indicative of the particular parameter of interest. In the case of a through-transmission type measurement, the radiation sensors would be positioned at the side of the film opposite the radiation source to detect radiation transmitted through the film.
Various gauges for accurately measuring the organic coating applied to a metal substrate have been considered. However, to be truly effective, the coating thickness gauge should desirably be capable of measuring the coating immediately after application, while it is still wet, so that immediate alterations in the coating operation can be made to vary the coating thickness if necessary. Since the coating is not in a dry, handleable state for several seconds after its initial application, measurements at a point after the coating is dry would permit the production of considerable improperly coated sheet prior to detection thereof in a coating line moving at several hundred feet per minute. Further, any accurate measurement of a wet, mobile, fluid coating must omit physical contact between the measurement device and the coating. Also, to insure no interruption in production, the measurement should be made while the coated sheet is moving.
A reflection-type, dual-beam infrared measurement system for determining film thickness is disclosed in U.S. Pat. No. 3,017,512 issued to H. J. Wolbert on Jan. 16, 1962. Wolbert's apparatus relies on passing infrared radiation through a film or coating carried on a reflective surface of an opaque substrate. The incident radiation is reflected from the surface and out through the coating. Means are provided for selecting from the emitted infrared radiation a beam of light, a portion of the energy of which is absorbed by the coating due to the chemical structure thereof, and another beam of the emitted radiation whose wave length is such that none of this energy is absorbed by the coating. Wolbert characterized the partially absorbed beam as a sample beam and the non-absorbed beam as a reference beam.
Wolbert, makes use of a known relationship between the absorption of radiant energy and the amount of absorptive material in the path of the radiant energy which may be characterized as follows: log of I.sub.O /I = kcd where; I.sub.O is the intensity of the reference beam striking the detector; I is the intensity of the sample beam striking the detector; k is a constant; c is the concentration of absorptive material in the sample; and d is the thickness of the sample. Wolbert has taught that while the above relationship was previously used mainly in determining the concentration of a certain substance in a sample by transmitting light through the sample of known thickness, the relationship is equally effective in determining thickness of a mobile, liquid film on an opaque but reflective substrate by passing the radiation into the coating and out again by reflecting the radiation from the substrate. Because there is a direct relationship between the thickness of a sample film and a difference between I.sub.O and I, the need to determine I.sub.O and I, separately in order to calculate d is obviated.
It has been found that infrared light is particularly applicable in the method and apparatus of Wolbert inasmuch as substantially all organic compounds will absorb infrafred radiation at specific frequencies within that range. The specific frequency or wave length at which the radiant energy is absorbed is a characteristic of the structure of the compound. The amount of radiant energy absorbed by the organic chemical is directly related to the quantity of absorbing chemicals in the path of the radiation.
One type of problem which occurs when measuring film thicknesses with a device such as is taught by Wolbert resides in the fact that Wolbert must chop or break up the radiation incident on the film to provide an AC output which per force must yield an average thickness determination for the whole region of the film moving past a given location in the time interval between successive pulses of light from the chopped reference and sample beams. Obviously, a particular readout may or may not provide a good indication of acceptable film thickness depending on whether or not the film being sampled is uniform.
Another problem which arises with either a through-transmission type measurement or a reflection type measurement such as is taught by Wolbert with respect to a film having specular surfaces is that there will be both first and second surface reflections which occur at the opposite surfaces of a self-supporting film or, in the case of a coated base sheet, at the respective outer or exposed surface of the film and the opposite surface at the interface of the film in the base sheet. While the first and second surface reflections produce respective signal components, these reflection components for each specific wave length suffer phase displacement which, depending on the thickness of the film, may interfere with one another and result in an output signal, in part, which is a function of this phase displacement. With one of the beams of radiation of a wave length selected to not exhibit a characteristic absorption with respect to either the film or base sheet and the other wave length selected to exhibit a characteristic absorption as to the film, it is evident that, at certain film thicknesses, variations in relative phase displacements of the refelected components will occur due to the interference phenomenon and produce corresponding variation in the detected signals with consequent error in the measurement.
Many attempts have been made in the prior art to solve the interference problem described above and typical of the suggested solutions are those found in U.S. Pat. No. 3,631,526 issued to Donald C. Brunton on Dec. 28, 1971. Briefly, Brunton utilizes a system similar to that of Wolbert, supra, wherein a first aspect of his solution Brunton directs each beam of radiation in a wide angle toward the surface of the film to be incident thereto at a relatively broad spectrum of angles rather than a single specific angle of incidence as in the case of Wolbert. This angle of incidence spectrum is selected to be of such breadth that reflection components will be added at all possible phase angles for each beam of radiation and the effect of interference between first and second surface reflection components for each beam is minimized. As a further refinement of his wide-angle technique, Brunton teaches selecting the reference and sample wave lengths to be sufficiently close together so that the relative phase displacement between the respective first and second surface reflection components will be minimal. Brunton teaches a third technique which may be utilized in combination with the wide-angle reflection technique or which may be utilized independently and which comprises the utilization of a relatively broad spectral band of wave lengths for the reference and sample radiation beams. Brunton teaches that utilization of a sufficiently broad spectral band of wave lengths will also result in the addition of reflection components at all possible phase angles with consequent minimization of interference error.
The prior art fails to disclose a system for simultaneously monitoring both thickness and uniformity of a rapidly moving film or such a system which is not adversely affected by interference error due to reflectance from the upper surface of the film. Non-uniformity of films frequently occurs as close together as one-quarter inch along the length of a strip moving at 500-600 or more feet per minute, and any system for monitoring such non-uniformity must have a rapid response time in order to be useful. On the other hand, monitoring of thickness will be more practically accomplished on the basis of a readout every one-third second or even less often. If thickness readouts were provided any more often, the result would produce a blur of number readings which would make utilization difficult. Accordingly, a system is desired for continuously monitoring the uniformity of a rapidly moving film and also separately and simultaneously monitoring the thickness of the film.